Overlapping community detection with igraph or other libaries

I would like to detect overlapping communities in small networks/graphs. By overlapping, I mean that a node can be included within more than one communities/clusters in the output of the detection algorithm.

I have looked at various community detection algorithms curretly provided by igraph , but I think none of them handles overlapping communities.

Ideally, I would like to be able to programmatically utilize some implementation of such algorithm(s) in Python. However, implementation in other languages is OK too.

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I have implemented the hierarchical link clustering algorithm of Ahn et al a while ago using the Python interface of igraph; see its source code here.

Also, implementing CFinder in Python using igraph is fairly easy; this is what I came up with:

#!/usr/bin/env python
from itertools import combinations

import igraph
import optparse

parser = optparse.OptionParser(usage="%prog [options] infile")
parser.add_option("-k", metavar="K", default=3, type=int,
        help="use a clique size of K")

options, args = parser.parse_args()

if not args:
    parser.error("Required input file as first argument")

k = options.k
g = igraph.load(args[0], format="ncol", directed=False)
cls = map(set, g.maximal_cliques(min=k))

edgelist = []
for i, j in combinations(range(len(cls)), 2):
    if len(cls[i].intersection(cls[j])) >= k-1:
        edgelist.append((i, j))

cg = igraph.Graph(edgelist, directed=False)
clusters = cg.clusters()
for cluster in clusters:
    members = set()
    for i in cluster:
        members.update(cls[i])
    print "t".join(g.vs[members]["name"])

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If you don't mind using another programming language, you have CFinder (Java), which is based on clique percolation (it basically looks for tightly connected cliques), OSLOM (C++), which optimizes a statistical measure, and certainly others.

Otherwise, if you want to stick to python, you could also apply the method by Evans & Lambiotte '09: 1) transform your graph into a line graph, 2) apply a regular community detection method on it, for example using igraph, and 3) obtain overlapping communities. Converting your graph into a line graph does not look too complicated, and should be fast, provided your original graph is not too large. It will be faster than performing the community detection itself, in any case.

Note there are alternatives methods to that of Evans & Lambiotte to get overlapping communities from a regular (mutually exclusive communities) method, such as those of Bennet et al. '12 or Wang et al. '09. However, implementing them seems less straightforward.

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According to this blog, networkx can now compute for overlapping communities.

The code below is for clique percolation method and is available in Networkx 11.6. Github here

import networkx as nx
from itertools import combinations

def get_percolated_cliques(G, k):
perc_graph = nx.Graph()
cliques = list(frozenset(c) for c in nx.find_cliques(G) if len(c) >= k)
perc_graph.add_nodes_from(cliques)

# Add an edge in the clique graph for each pair of cliques that percolate
for c1, c2 in combinations(cliques, 2):
    if len(c1.intersection(c2)) >= (k - 1):
        perc_graph.add_edge(c1, c2)

for component in nx.connected_components(perc_graph):
    yield(frozenset.union(*component))

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